1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of synthesizing a full color image without causing deviations of color layers by adjusting timings for writing color layers.
2. Description of the Related Art
A background color image forming apparatus using an electrophotographic method forms a color toner image by superimposing toner images in four colors (i.e., yellow, magenta, cyan, and black) onto an intermediate transfer member and transferring the superimposed toner images onto a transfer sheet.
A two-station type color image forming apparatus is one example of the background color image forming apparatus, and includes an intermediate transfer member and two image forming units arranged to oppose a moving surface of the intermediate transfer member. Each one of the image forming units is configured to form toner images with two different color toners from among the four colors, and includes an image carrier, an image writer, two development mechanisms, and a switcher. The image writer forms an electrostatic latent image on the image carrier, and the development mechanism visualizes the electrostatic latent image on the image carrier with toner to form a corresponding color toner image. The switcher switches between the two development mechanisms to form two color toner images. The intermediate transfer member is driven to move so as to receive the four color toner images from the two image forming units. Thereby, the four color toner images are transferred and superimposed as a color toner image at a transfer position on a surface of the intermediate transfer member.
In the above-mentioned process, however, if the transfer positions of the four color toner images on the surface of the intermediate transfer member are deviated, the four color toner images are not accurately transferred at a position on the surface of the intermediate transfer member. As a result, an output superimposed color image shows deviations of color image elements and has an inferior image quality. This problematic phenomenon is referred to as a color deviation in the following discussion.
To attempt to eliminate the above color deviation, the color image forming apparatus needs to perform a control operation for starting the transfer of the four color toner images on the surface of the intermediate transfer member at a common start position. Such a control operation is performed typically with a reference mark provided to the intermediate transfer member and a detector for detecting the reference mark. The color image forming apparatus generates sync clock signals and counts a number of sync clock signals generated during a time period after the detector detects the reference mark until the image writer starts writing an electrostatic latent image onto the image carrier. Based on the number of sync clock signals counted, a distance between the reference mark and the position of starting the image transfer on the surface of the intermediate transfer member is determined.
Another control operation for attempting to eliminate the color deviation forms test patterns with respective color toner on the intermediate transfer member, optically detects the test patterns, and determines positional deviations of the test patterns. Based on such positional deviations determined, the positions of images are corrected.
Another control operation for attempting to eliminate the color deviation uses a plurality of reference marks formed on the intermediate transfer member. This attempt measures a time period between detections of two adjacent reference marks and compares it with a predetermined time period to determine whether a time deviation occurs due to variations in load. When the time deviation is determined as occurring, a revolution speed of the image carrier is adjusted to an appropriate speed.
As another example, a tandem type color image forming apparatus includes an intermediate transfer member and four image forming units arranged to oppose a moving surface of the intermediate transfer member and each having at least one reference mark. This apparatus performs a transfer of a first color toner image onto the intermediate transfer member based on a detection of the reference mark and transferring second through fourth color toner images onto the intermediate transfer member based on the number of sync clock signals counted on and after the detection of the reference mark. In this case, the image transfer to the intermediate transfer member is merely repeated for the first through fourth color toner images, regardless of a size or a length of the transfer sheet or a number of prints.
In the two-station type color image forming apparatus, however, the order of transferring in terms of color needs to be changed in accordance with, for example, a number of transfer sheets placed on the intermediate transfer member. This is because a maximum number of transfer sheets placeable on the intermediate transfer member varies depending upon the lengths of the transfer sheets.
Further, the adjacent image forming units in the two-station type color image forming apparatus are arranged with a distance greater than that between the adjacent image forming units in the tandem type color image forming apparatus. Accordingly, the two-station type color image forming apparatus needs to move the intermediate transfer member for a longer distance and that may increase the positional deviation of the superimposed color toner images due to variations in moving speed of the intermediate transfer member.